Ordnance datum

An ordnance datum or OD is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as AOD for "above ordnance datum". Usually mean sea level (MSL) at a particular place is used for the datum. In particular:

• In Great Britain, OD for the Ordnance Survey is ODN (Ordnance Datum Newlyn), defined as the MSL as recorded by the Newlyn Tidal Observatory between 1915 and 1921.
  • Prior to 1921, OD was defined as MSL as recorded in the Victoria Dock, Liverpool, during a short period in 1844 (ODL).
• In Northern Ireland, OD for the Ordnance Survey of Northern Ireland is Belfast Ordnance Datum, the MSL at Clarendon Dock, Belfast, between 1951 and 1956.
• In Ireland, OD for the Ordnance Survey of Ireland is Malin Ordnance Datum: the MSL at Portmoor Pier, Malin Head, County Donegal, between 1960 and 1969.
  • Prior to 1970, Poolbeg Ordnance Datum was used: the low water of spring tide at Poolbeg Lighthouse, Dublin, on 8 April 1837. Poolbeg OD was about 2.7 metres (9 feet) lower than Malin OD.